Richard Schiff as Toby Ziegler in Underneath the Lintel. Or something like that. Photograph: Tristram Kenton

Suspend your disbelief. Many regard this principle as the cornerstone of theatre, as if the whole representational enterprise depends on turning off, in addition to your mobile phone, some otherwise essential part of your brain before taking your seat. One of the staples of the suspend-your-disbelief brigade is the idea that, in order to believe in a character, you have to forget about the actor playing it: the actor must "become" the character. Having long suspected the inanity of this, I had the opportunity to confirm it on seeing Glen Berger's intriguing if sometimes clumsy take on the Wandering Jew myth, Underneath the Lintel, starring former director of communications to President Jed Bartlett Toby Ziegler.

OK, so the actor is actually Richard Schiff. But the man I saw coming on to the stage of the Duchess theatre, clutching a cumbersome suitcase and nursing an awkward manner, was Toby Ziegler, looking rather worse for wear after his implausible, ugly ejection from the White House. When he opened his mouth, though, the (very creditable) Dutch accent soon reminded me that this was no hot shot from the West Wing, but a semi-autistic librarian from Hoofddorp, Amsterdam, about to report on his attempts to track down the borrower of a Baedeker, returned 113 years too late. And as it dawned that this man was engaged in a pursuit as eternal and hopeless as that of his mythical quarry, a number of things struck me, the foremost of which was: what would Toby Ziegler make of this?

I pictured him back in his Washington office, breathing heavily through his moustache at the playwright and saying, "What do you want me do with this crap?", a slight wavering perceptible in the otherwise monotone, even delivery of his continuing condemnation. "I don't know how you'd expect me to sell this stuff. No one's buying limitless confusion right now, and I sure as hell don't see any candy on the counter. There's nothing about this that doesn't stink." Unrelenting, but a brief touch on the writer's arm on the return of the script would let him know that his efforts should continue.

The picture of Ziegler helped me considerably in appreciating the play. If he believed in it, it must be worth pursuing, worth believing in too. Talking to Schiff later that evening, I decided against calling him Toby, but did ask him how he found the play. "The play's not perfect", he said, more kindly in manner than Toby, "which is hard because the actor has to carry the play. With a really great script, the play carries the actors, but with this you have to be continually at your best, supplying all the energy, keeping it going."

He thought the evening's performance had been ok, not as bad as on press night, but he hadn't felt strong enough to carry this flawed, ungainly but searching and sometimes searingly insightful enquiry, which he hadn't been able to put down when he first read it while still playing Toby, and which saw him moving from a big-league TV series to a one-man show in a town in New Jersey, where it got rave reviews, and here, now, in London, where it didn't.

I told him that I had liked the way his West Wing character rubbed off on the hopeless Dutch librarian, that this had added to the play. He didn't wince. I told him that the suspension of disbelief is a fallacy, as stupid as the idea of looking at a figurative painting without attending to the marks on the surface, the traces of effort and intention that leave their impression deep in the image; that when you're in the theatre, you pull every resonant feature into the experience and try to make it stick. Schiff smiled and said he saw my point. I can't see Toby Ziegler having been so forbearing. "There's nothing about this that doesn't stink".

But Toby Ziegler was the reason I went to see the play, and Toby Ziegler was part of why I found it worth wrestling with. As Bartlett's director of communications, Ziegler was a thorny inspiration to those around him, an astringent reminder of why they may never rest in the search for a political solution. As an actor in a play about a restless search for absolution, he was an equally thorny reminder that some questions are worth the effort to ask.